Insulation-liquid-filled electrical equipment, such as oil-filled disconnectors, circuit breakers, and especially transformers such as power and distribution transformers (and/or instrument transformers), are filled with insulation liquid, such as oil, for cooling and electrical insulation purposes. Faults inside the electrical equipment as well as degradation of the insulation liquid and of other insulation components such as insulation paper provided within the electrical equipment can form larger amount of gasses than under normal conditions, which dissolve into the liquid. Hence, measuring the dissolved gas concentration gives information about the “health condition” of these electrical equipment.
Transformers and the other electrical equipment mentioned above are important components of the electrical grid, and their failure can be very costly. A transformer is supposed to operate continuously and as error-free as possible over many years or even decades. Hence, it is important to detect faults, malfunctions and degradation, so that errors that may eventually cause failure of the electrical equipment can be detected in time to take appropriate counter-measures.
As already mentioned, faults in insulation liquid-filled transformers and other electric equipment can be accompanied by the development of larger amounts of gases dissolved in the insulation liquid. The quantity and composition of the decomposition gases is dependent on the underlying defect: A large fault with high energy content, such as rapid overheating or arcing, causes large amounts of gas to be produced in a short period of time, whereas the amount of gas produced by a small fault may be relatively smaller.
According to the IEEE Guide for the Interpretation of Gases Generated in Oil (IEEE C57.104), the status conditions (risk or fault condition) can be classified in transformers according to four conditions that depend on the concentration of dissolved gases. Table 1 shows hydrogen concentration according to the respective classified conditions.
TABLE 1Status conditionHydrogen (H2) content in insulation liquid (ppm)status condition 1≦100 status condition 2101-700 status condition 3701-1800status condition 4 >1800
Thus, if the nature and amount of individual gases dissolved in the insulation liquid are known, this information can be used to identify the type and severity of the corresponding electrical fault in the equipment, e.g. according to these standardized health conditions 1 to 4.
To verify the health status of the insulation liquid of such electrical equipment, two main methods are known: According to a first known method, also referred to as the offline-method, samples of the insulation liquid are regularly (e.g. yearly) taken on-site and analyzed in a specialized laboratory by dissolved gas analysis. However, this offline-method can be burdensome and does not allow obtaining real-time data, and is of no further interest here even though it is a widely used method.
According to a second known method, also referred to as online-method, measurements monitor the gas concentration in the insulation liquid directly and (quasi-)continuously. These on-line sensors include semiconductor sensors, thermal-conductivity analyzers, pellistors and fuel cell sensors, among others. These sensing techniques can involve a complicated gas separation system that adds complexity and cost to the sensor design and calibration.
However, even though the known online systems allow detailed hydrogen concentration values to be obtained, some drawbacks and obstacles remain, such as complex sensor design, problems due to sensor aging and drift, an issue of calibrating and periodically re-calibrating the sensor, high cost, high maintenance requirements and/or limited life-time reliability of the sensors.
Optical hydrogen sensors to be used in transformer oil were previously investigated by M. Slaman, R. Westerwal, H. Schreuders, B. Dam [Proc. SPIE Vol. 8368 836805-1, 2012] and by M. A. Butler, R. Sanchez, G. R Dulleck [Sandia Report Sand 96-113]. In both reports it is proposed to develop a continuous hydrogen sensor that has an almost linear or continuous optical output over a whole hydrogen concentration range.
Another optical hydrogen sensor to be tested in transformer oil was investigated by GUO-MING M A ET AL: “High sensitive and reliable fiber Bragg grating hydrogen sensor for fault detection of power transformer”, SENSORS AND ACTUATORS B: CHEMICAL: INTERNATIONAL JOURNAL DEVOTED TO RESEARCH AND DEVELOPMENT OF PHYSICAL AND CHEMICAL TRANSDUCERS, ELSEVIER S.A, SWITZERLAND, vol. 169, 20 Apr. 2012 (2012 Apr. 20), pages 195-198, XP028520709, ISSN: 0925-4005. The optical sensor uses a fiber Bragg grating (FBG) sheathed with an intermediate polyimide and Ti layer and an outermost Pd layer to absorb hydrogen. The absorbed hydrogen induces a strain change on the FBG which results in a continuous wavelength shift response to hydrogen concentration.